Pt100 is usually divided into two-wire, three-wire and four-wire system three.
General short-distance selection of two-wire connection method, the distance selection of three-wire connection method, requiring high precision, close selection of four-wire connection method. The benefits of the two-wire system than the two-wire system can compensate for the deviation of the line resistance, and the anti-jamming is not a concept. There are many advantages and disadvantages of the three different strengths and weaknesses, two lines can not eliminate the impact of wire resistance. The four-wire system eliminates the effects of wire resistance. Four-wire PT100 has two lines for the measurement, the other two are used for compensation, four-wire PT100 has two lines (one at each end of the thermal resistance) is to provide current, the other two Is the collection of voltage. Which circuit should be used in accordance with the specific requirements of the system, if the accuracy requirements in general, the use of three lines is economical, stable and practical choice
Due to the development of microprocessors, the nonlinearity of Pt100 can be corrected. Therefore, most of Pt100 sensors adopt four-wire measurement method (non-bridge method). The principle is as follows:
Pt100 voltage U1 = ISRt. IS is constant current, Rt is Pt100 resistance.
Lead L1, L2 resistance will affect the measurement results, for this, the L1, L2 port signal input high input electrical impedance (> 1012Ω), differential amplification, so L1, L2 current ≈ 0, L1, L2 resistance negligible , So there are Ui = U1. This also eliminates the lead resistance.
In the bridge circuit, in order to reduce the thermal resistance of the resistance with the temperature changes on the tributary current and limit the current flowing through the thermal resistance, the composition of the two branches of the upper resistance of the bridge is usually taking the thermal resistance of dozens of Times, its value to 10-50K (and bridge power supply voltage), the next resistance and thermal resistance of a temperature resistance of the same temperature. Measure the potential difference between the two. Even so, the thermal resistance value with the temperature changes on the branch current or will cause the output of the nonlinear, usually need to do some compensation.
If the resistance is measured directly, the constant current source should be used to supply the thermal resistance. When the thermal resistance value changes, the branch current will be kept constant and the thermal resistance voltage drop will be linearly better temperature function.
Before the amplification should do the filtering process or in the amplifier circuit plus integral components.
How to determine the quality of pt100, with a multimeter to measure it?
According to the index table reference temperature at that time to see whether the match value
This is usually the case, a reference voltage will be added to the pt100 circuit, measuring pt100 on the voltage signal (mv), the resistance change is the voltage signal naturally changes, and then after the amplifier into the A / D chip A / D conversion, after the program and then converted into a voltage signal voltage value, the use of look-up table (the resistance value and the corresponding temperature values into the form of the chip rom) to the temperature value.
Analog thermocouple test
The most accurate method is to use the resistance box, and more than one way to slowly. The thermocouple uses a millivolt meter to simulate the output of the secondary meter, and the millivolt meter can also measure the thermocouple. These are not difficult, the difficulty is to establish a standard constant temperature field.
The relationship between voltage and temperature is generally non-linear, for 8-bit microcontroller or look-up table method is good
(3 lines), the output, the power supply is not isolated for the three-wire system, the equipment is in the control room or the sensor; input (3) Root line), (output, power sharing 2 lines) three is not isolated for the two-wire system, the equipment in the sensor, for the integration.